1. Field of the Invention
The present invention concerns a method to acquire measurement data of a breathing examination subject by means of magnetic resonance technology and an associated computer program
2. Description of the Prior Art
Magnetic resonance technology (in the following the abbreviation MR stands for magnetic resonance) is a known technique with which images of the inside of an examination subject (for example) can be generated. Expressed simply, in an MR examination one or more MR measurements (data acquisition) are performed with the examination subject in a comparatively strong, static, for the most part homogeneous basic magnetic field (field strengths from 0.2 Tesla to 7 Tesla or more) in an MR apparatus, so that nuclear spins in the subject orient along the basic magnetic field. The basic magnetic field is also termed B0 field. Radio-frequency excitation pulses are radiated into the examination subject to trigger nuclear magnetic resonances, the triggered nuclear magnetic resonances are measured and MR images (for example) are reconstructed based thereon. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored in a k-space matrix as complex numerical values. An associated MR image can be reconstructed by means of a multidimensional Fourier transformation from the k-space matrix populated with such values.
In magnetic resonance imaging (“MRI”) the breathing movement of a patient to be examined by means of MR can lead to artifacts known as ghosts (“ghosting”), and/or blurring and/or to intensity loss in the generated images, primarily in an examination of the organs of the thorax and the abdomen, thus of examination regions affected by the breathing movement of the patient. Additionally the breathing motion can lead to registration errors between generated images. These artifacts can hinder a finding on the basis of these images (for example by a physician) and can lead to the situation that lesions (for example) are overlooked.
Numerous techniques exist in order to reduce artifacts resulting from a breathing movement. Two groups of these techniques are known as respiratory gating and respiratory triggering, with these two terms not always being clearly separated.
Respiratory gating is an MR measurement during which the breathing of the patient is detected and associated with the acquired measurement data, wherein the repetition rate of the MR measurement (in particular its TR, thus the time between the successive excitation of a slice) is independent of the breathing of the patient. Rather, the repetition rate is controlled by a parameter or by an additional, different physiological signal, for example an ECG. The breathing information is then used (for example) to repeatedly acquire particular measurement data (packets)—that, for example, were acquired during strong breathing movement—until they have been acquired in a more quiescent phase of the breathing cycle. Another use of the breathing information can be to acquire k-space lines that are expected to be particularly movement-sensitive or k-space lines determining the image impression in an exceptional (quiet) breathing phase (what is known as “ROPE”—“respiratory ordered phase encoding”).
Respiratory triggering is a technique that synchronizes an MR measurement (an imaging MR measurement, for example) with the breathing of the freely breathing patient and attempts to acquire defined packets of measurement data only during a marked phase of the breathing cycle. The marked phase is for the most part the relatively quiet phase of the breathing cycle at the end of the expiration. The acquisition of the measurement data is thus triggered by the phase of the breathing cycle. If a specific slice is hereby excited only once per trigger, the effective repetition rate (TR) of the measurement sequence is thus a whole-number multiple (v=1, . . . , k) of the mean breathing cycle of the patient.
Respiratory gating and respiratory triggering are described in the article “Comparison of Respiratory Triggering and Gating Techniques for the Removal of Respiratory Artifacts in MR Imaging” by Lewis et al., Radiology 1986; 160:803-310, for example.
Furthermore, there are breath hold techniques in which the patient must hold his or her breath for the duration of the acquisition of the measurement data in order to avoid movement artifacts. One example of a repeated breath hold technique is described in the article by Wang et al., “Navigator-Echo-based Real-Time Respiratory Gating and Triggering for Reduction of Respiratory Effects in Three-dimensional Coronary MR Angiography”, Radiology 1996; 198:55-60.
In the following respiratory triggering is discussed in detail. As noted, the respiratory triggering attempts to synchronize an MR measurement with the breathing cycle of a freely breathing patient and tries to limit the acquisition of image data to the relatively quiet phase of the breathing cycle at the end of the expiration. For this the breathing of the patient is detected as a physiological signal, for example, with a pneumatic sensor. After an initial learning phase of the trigger algorithm, a “trigger” is generated by the trigger algorithm as soon as the trigger algorithm detects a predetermined event. Initiated by such a “trigger”, an MR sequence is executed that acquires an (again predetermined) portion of the measurement data from one or more slices. After this predetermined measurement data packet is acquired, the acquisition of the measurement data stops automatically until the trigger algorithm generates the next trigger. The second measurement data packet is thereupon acquired.
This workflow is continued until all measurement data of all slices are acquired. The predetermined event whose detection generates the trigger occurs when, for example, the last measured physiological signal crosses a threshold (set by an operator of the MR system) between the physiological signal (averaged over multiple breathing cycles) during maximum inspiration and the averaged physiological signal during maximum expiration. Triggers are normally generated only during the exhalation. In order that the acquisition of the measurement data can be limited to the relatively quiet phase of the breathing cycle at the end of expiration the following two conditions therefore must be fulfilled: on the one hand, a suitable threshold must be selected and on the other hand the acquisition duration of the predetermined measurement data packet must be brief relative to the individual breathing cycle of the patient.
The breathing cycle of the patient is typically between 3 and 6 seconds but is subject to significant individual (as well as illness-dependent) fluctuations. The acquisition duration of the measurement data packet depends on a number of parameters of the pulse sequence used for acquisition. In particular, in what is known as a multi-slice measurement, in which measurement data of different slices are acquired within one TR interval—on the number of slices from which measurement data are acquired after a trigger (i.e. within one measurement data packet). A very short acquisition duration per measurement data packet relative to the breathing cycle of the patient thereby reduces the efficiency (i.e. extends the total examination duration) since the number of breathing cycles that are needed in order to acquire all measurement data packets of all slices increases with decreasing amount of measurement data per breathing cycle. In the aforementioned example of a multi-slice measurement the efficiency therefore decreases with the decreasing number of slices from which measurement data are acquired per breathing cycle.
In contrast to this, if the acquisition duration per measurement data packet exceeds the duration of the relatively quiet phase at the end of the expiration, the acquisition of the measurement data also ensues during the subsequent inspiration. This can in turn lead to breathing artifacts or misregistration between slices. If the acquisition duration per measurement data packet reaches the duration of the breathing cycle or even goes beyond this, a trigger can, moreover, not be generated in every breathing interval but rather only in every second breathing interval, for example. This again increases the total examination duration and thus reduces the efficiency of the measurement.
In the prior art it is the task of an operator of an MR system to adapt parameters of the imaging MR sequence of an examination to the individual breathing cycle of a patient to be examined on the one hand and to input additional parameters on the other hand that describe the event that should initiate a trigger.
For example, in MR systems from Siemens it is known for an operator to first establish an acquisition window depending on a determined breathing cycle of the patient. The acquisition window is a time interval that upwardly limits the acquisition duration of measurement data per trigger. Values of parameters of an MR sequence that is to be used, such as the number of slices, repetition rate TR, turbo factor etc. are then limited such that the acquisition duration per trigger does not exceed the acquisition window. In Siemens MR systems the event that initiates the trigger is characterized by a parameter that describes a percentile threshold between the previously calculated average signal during maximum inspiration and the likewise pre-calculated average signal during maximum expiration. If a last measured physiological signal (breathing signal) exceeds this value, a trigger signal is initiated. The placement of the acquisition window is described in the Application Brochure for Body Imaging by Siemens AG in the chapter, “Application: Respiratory gating”, Pages 110-113, for example. It is noted that the method designated there with “Gating” is a “triggering” according to the definition given above.
The approach in a Philips MR system is very similar to that by Siemens. There an acquisition window is likewise established by an operator depending on the determined breathing cycle of the patient. This is described in Chapter 2.24.2 “Respiratory triggering” on pages 2-46 through the top of 2-48 in the “Application Guide, Volume 2, Intera, Achieva, Panorama 1.0T, Release 1.5”, for example.
In an MR system from GE an acquisition window but also a trigger position is to be set (among other things) as parameters for the implementation of a respiratory-triggered measurement. More detail is described in the chapter “Respiratory Gating and Triggering Parameters” on Pages 45-32 through 45-34 in “MR 1.5 Signa® EXCITE™ 11.0 Learning and Reference Guide” by GE, for example.
All leading manufacturers assist the operator in the task that was just described in that they visualize the measured physiological signal as a function of time and calculate and display the averaged breathing cycle of the patient. For example, the average breathing cycle is thereby defined as the average time interval between two successive extremes of the physiological signal during maximum inspiration.
Prerequisite for a respiratory triggered and respiratory gated MR measurement is the detection of the breathing movement as physiological signal. For this purpose breathing belts or cushions can be used, for example, that are placed on the patient and that detect (for example by means of pneumatic sensors) the rise and fall of his ribcage that are caused by the breathing.
An additional possibility for the detection of the breathing signal is the use of navigators. These are MR signals that are generated and received by means of the MR apparatus being used in addition to the actual MR signals for the desired MR examination (for example an imaging or spectroscopic examination). A navigator is normally a short sequence that, for example, acquires MR signals of the diaphragm from which (for example) the position of the diaphragm of the patient at a point in time of the navigator acquisition can be extracted. The diaphragm position can then be used as a physiological signal corresponding to a current breathing movement at the point in time of the navigator acquisition. The navigator sequence is interleaved with the imaging sequence and the breathing phase or position that is determined with the navigator measurement is assigned to the anatomical MR data directly acquired after said navigator sequence. This analogously applies for spectroscopic examinations.
However, given the use of navigators for respiratory triggering the problem results that the breathing cycle of the patient is generally still unknown during the measurement preparation since the breathing of the patient is first detected during the MR measurement. For an operator it is thus impossible to adapt the imaging parameters of the MR sequence to the individual breathing cycle of the patient, for example, or to optimally set the parameters that describe the desired trigger event before the beginning of the MR measurement.
In implementations of respiratory triggering with navigators in MR systems from Siemens AG the acquisition duration per trigger is graphically visualized, together with the detected physiological breathing signal, for the operator during a learning phase. The operator thereby has the possibility to terminate the MR measurement early given an unsuitable parameterization and to adapt it accordingly. This is described on Pages 20-27 in the aforementioned Application Brochure for Body Imaging by Siemens AG in the Chapter “Measurement during normal breathing—Navigator gating with 2-D PACE (I-IV)”. However, in order to implement this procedure correctly a good training of the operator is required so that she has sufficient knowledge about the connections [relationships] of the parameterization and the breathing cycle.
Furthermore, in Siemens MR systems with what is known as “scout mode” the possibility is provided to implement a short MR measurement in which only the navigator sequence is deployed. The physiological breathing signal is visualized during this pure navigator measurement. As soon as a complete breathing period has been detected, the breathing cycle is calculated and displayed. The breathing cycle of the patient would therefore be known in turn during the preparation of the imaging sequence (for example) following the “scout mode” and can be used by the operator in setting the parameters of the imaging sequence. This is described on Page 108 in the aforementioned Application Brochure for Body Imaging by Siemens AG in the chapter “Application Navigator Gating—Optional measurement parameters (II)”, for example.
Without an adaptation of the parameters of the MR measurement—thus the parameters that establish the type of acquisition of measurement data by means of magnetic resonance—only unsatisfactory results can be achieved for the most part. Particularly given patients in which measurement data from many slices are required to cover the organ to be examined, this frequently leads to the situation that the acquisition duration per trigger is significantly longer than the breathing cycle. As explained above, this leads to images with artifacts and to long examination times.